1. Field of the Invention
The present invention relates to a power source apparatus using a piezoelectric transformer, and an image forming apparatus thereof.
2. Description of the Related Art
In an image forming apparatus which forms an image in an electronic photographing process, if a direct transfer method is employed to enable a transfer unit to be in contact with a photoreceptor to transfer an image, the transfer unit uses a conductive rubber in the form of a roller having a rotation axis as a conductive body. In this case, driving of the transfer unit is controlled according to a process speed of the photoreceptor. Also, a direct current (DC) bias voltage is used as a voltage to be applied to the transfer unit, and a polarity of the DC bias voltage is made the same as that of a transfer voltage via a corona discharge method.
Thus, in order to perform reliable transfer of the image by using the transfer roller, a voltage of approximately 3 kV (with required current of microamperes) should be applied in general. According to a typical technique, in order to generate a high voltage required for image formation, a coil-type electronic transformer has been used. However, the coil-type electronic transformer is formed with copper wires, bobbins, and magnetic cores. When the coil-type electronic transformer is used to apply a voltage of approximately 3 kV, an output current value thereof is very low (i.e., microamperes), and a leakage current in each part should be minimized. In order to minimize the leakage current, a method of making the coil of the coil-type electronic transformer with a mould made of an organic insulating material has been used. However, when this method is used, there are risks of overheating to produce smoke and fire, and a relatively larger electronic transformer compared to a supplied power is required. Accordingly, it is difficult to reduce the size and weight of the coil-type electronic transformer.
In order to solve the above problems associated with the coil-type electronic transformer, a method of generating a high output voltage using a slim, light-weight, and high-output piezoelectric transformer has been considered. That is, if a piezoelectric transformer using a ceramic material is employed, it is possible to generate a high output voltage with an efficiency equal to or higher than that of the coil-type electronic transformer. Furthermore, since the piezoelectric transformer is irrespective of combining of a primary side thereof and a secondary side, putting a distance between electrodes of the primary side and the secondary side is possible, mould processing is not necessary, and risks of producing smoke and fire disappear. As a result, a method of generating a high output voltage by using the piezoelectric transformer enables a small-size and light-weight high-voltage power source apparatus to be made.
However, in the high-voltage power source apparatus using a piezoelectric transformer, frequency control is performed by an analogue signal processing circuit, and therefore, a frequency control operation is unstable. That is, if a control voltage is rapidly changed in order to quickly raise (or drop) an output voltage of the high-voltage power source apparatus, a resonant frequency of the piezoelectric transformer is exceeded and it is impossible to control the output voltage. Also, when power equal to or greater than a capacity of a piezoelectric transformer is required due to an unpredicted situation, a driving frequency of the piezoelectric transformer exceeds the resonant frequency, and it is impossible to control the output voltage of the piezoelectric transformer, to thereby cause bad image signals to be generated. Also, even as the output voltage from the piezoelectric transformer cannot be controlled, oscillation of a circuit operation in the high-voltage power source apparatus occurs.
In addition, a delay of a rise time occurs due to a spurious driving frequency from a time when a control signal to turn on the high-voltage power source apparatus is input to an operational amplifier to a time when a desired set output voltage value is reached.
Furthermore, as a plurality of resonant frequency points relative to the output voltages exists for a piezoelectric transformer, in order to increase a range of the output voltages that may be output by the piezoelectric transformer, frequencies in a range that result in low efficiency voltage output would be used, which decreases the overall efficiency of the high-voltage power source apparatus as a whole.
In addition, the driving frequency of the piezoelectric transformer is changed too much from a primary resonant frequency in order to reduce the output voltage, the driving frequency approaches a next resonant frequency, and as a result, the output voltage will start to increase after reaching a minimum voltage. For this reason, it is difficult to achieve a low voltage output of the high-voltage power source apparatus using the piezoelectric transformer.